1. Field of the Invention
This invention refers to a new method of obtaining ultra fine magnetic Nd-Fe-B particles of various sizes. This method is based on carrying out the particle formation reaction in the sine of micro-reactors, in such a way that the volume of these restrict the maximum size of the particles to be formed, in addition it being able to obtain various particle sizes by modifying the size of the micro-reactors used for the reaction process.
In order to obtain particles of the desired size, it is necessary to use micro-reactors with a homogenous and easily changeable size. These characteristics are present in micro-emulsions.
Micro-emulsions are thermodynamically stable systems, formed by at least three components; two immiscible substances (usually water and oil) and a third component acting as a surface-active or amphiphile agent, able to solubilise the two former substances. The surface-active agents are molecules having a polar part (head) and an apolar part (tail), due to which they are able to solubilise two immiscible substances such as water (polar) and an oil (apolar).
2. Description of the Related Art
From a microscopic view, micro-emulsions are micro-heterogeneous systems with structures dependent on the water/oil ratio, by means of which they are classified into two types of micro-emulsion. The oil/water (oil in water) micro-emulsions are those containing a greater amount of aqueous solution and structurally they are formed by micro-drops of oil surrounded by the amphiphile molecules submerged in the aqueous medium (FIG. 1). The w/o (water in oil) micro-emulsions have a greater proportion of oil and from a microscopic view consist of dispersed aqueous micro-drops surrounded by molecules of amphiphile in the sine of the oil (FIG. 2).
The size of the micro-drops is dependent on the composition of the micro-emulsion and, for a specific micro-emulsion, variation occurs with temperature changes. See H. F. Heicke, Micro-emulsions, ed. I. D. Robb, page 17 (Plenum Press, N.Y., 1982); P. D. I. Fletcher, B. H. Robinson, F. Bermejo-Barrera and D. G. Oakenfull, Micro-emulsions, ed. I. D. Robb, page 221 (Plenum Press, N.Y., 1982); B. H. Robinson, Ch. Toprakcioglu, J. A. Dore and P. Chieux, J. Chem. Soc. Faraday Trans. I. 80, 413 (1984); J. S. Huang, S. T. Milner, B. Farago and D. Richter, Phys. Rev. Let. 59, 2600 (1987); M. Kotlarchyk, R. B. Stephens and J. S. Huang, J. Phys. Chem. 92, 1533 (1988); A. N. North, J. C. Dore, A. Katsikides, J. A. McDonald and B. H. Robinson, Chem. Phys. Let. 132, 541 (1986); and G. Fourche, A. M. Bellog and S. Brunetti, J. Colloid, Interface Sci. 88, 302 (1982).
Given that the magnetic Nd-Fe-B particles are formed by means of a chemical reaction in an aqueous medium, the aqueous micro-drops have a w/o micro-emulsion which comprise ideal micro-reactors to obtain such particles. If the reagents are ionic or polar, they will only be seen in the aqueous solution forming part of the micro-emulsion. The reaction will only take place within the aquous micro-drop and its volume will restrict the size of the final particle. The reaction produces a crystallization nucleous inside the micro-drop, which continues to grow by means of agglomeration until it forms a final micro-particle of a size approximately equal or less than the size of the micro-drop (FIG. 3).
For a specific composition and temperature, the micro-emulsions are formed by micro-drops of homogenous volume and, therefore, the particles obtained by a micro-emulsion reaction will also be of homogenous size. The size of a micro-emulsion's micro-drops can be varied by modifying its composition or, simply, its temperature. In this way, it is possible to avail of the adequate micro-reactors to obtain the micro-particles of the desired radius.